Circuit board and method for producing the same

ABSTRACT

An etching treatment is applied to a metal plate in accordance with a predetermined circuit pattern to form a circuit. Subsequently, a sandblast treatment is applied to an entire surface including the circuit to remove a conductive reactive layer remaining at a metal-removed portion of the circuit. Accordingly, an insulating substrate as an underlying layer is exposed from the metal-removed portion of the circuit. The sandblast treatment is performed under a condition in which an Ni plating layer remains on the circuit at a stage at which the conductive reactive layer remaining at the metal-removed portion of the circuit is removed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a circuit board which has acircuit on an insulating substrate, and a method for producing the same.The present invention relates to a circuit board which is preferablyused, for example, as a circuit board for cooling an electronic circuitchip composed of a semiconductor or the like, and a method for producingthe same.

[0003] 2. Description of the Related Art

[0004] It is important for a general circuit board for mounting thereona semiconductor device to efficiently transmit the heat generated by thesemiconductor device to the outside. In other words, the heat is a greatenemy for the semiconductor device. It is necessary that the internaltemperature does not exceed the maximum permissible joining temperature.In a semiconductor device such as a power transistor or a semiconductorrectifying device, the electric power consumption per operation area islarge. Therefore, the generated amount of heat is releasedinsufficiently with only an amount of heat released from the case(package) and the lead of the semiconductor device. The internaltemperature of the device may be raised, resulting in thermaldestruction.

[0005] A semiconductor device which carries CPU suffers the same problemas described above. The generated amount of heat is increased as theclock frequency is improved. Therefore, the thermal design inconsideration of the heat release is important.

[0006] According to the thermal design of preventing the thermaldestruction, the circuit board design and the mounting design areperformed for securing a heat sink having a large heat release area tothe case (package) of the semiconductor device.

[0007] Explanation will now be made with reference to FIG. 36 for aconventional circuit board 200 to which a heat-control measure isapplied (see, for example, Japanese Laid-Open Patent Publication No.11-307696).

[0008] As shown in FIG. 36, the circuit board 200 comprises a metal baseplate 202 for releasing the heat generated by a semiconductor chip, aceramic plate 206 for insulating the semiconductor chip 204 from themetal base plate 202, a circuit 210 of a metal plate 224 disposed on theupper surface of the ceramic plate 206 with a brazing member 208interposed therebetween, a lower electrode plate 214 disposed on thelower surface of the ceramic plate 206 with a brazing member 212interposed therebetween, a metal spacer 216 for widening the distancebetween the metal base plate 202 and the ceramic plate 206, a brazingmember 218 for securing the metal spacer 216 to the metal base plate202, a solder layer 220 for securing the semiconductor chip 204 onto acircuit 210, and a solder layer 222 for securing the lower electrodeplate 214 onto the metal spacer 216.

[0009] In the conventional circuit board 200 shown in FIG. 36, when thecircuit 210 is formed on the ceramic plate 206, the metal plate 224 isfirstly joined onto the ceramic plate 206 using the brazing member 208.Next, the metal plate 224 is selectively etched to form the circuit 210having a predetermined circuit pattern. The technique for forming thecircuit 210 by etching the metal plate 224 is disclosed, for example, inJapanese Laid-Open Patent Publication Nos. 8-97554, 9-181423, and7-235750.

[0010] A brazing member containing an active metal is used as thebrazing member 208 for joining the metal plate 224 onto the ceramicplate 206. In this case, it is possible to improve the joining strengthbetween the ceramic plate 206 and the circuit 210.

[0011] However, when the ceramic plate 206 and the metal plate 224 arejoined to one another using the brazing member 208 as shown in FIG. 36,a conductive reactive layer 226 is formed between the ceramic plate 206and the metal plate 224 as shown in FIG. 37. The conductive reactivelayer 226 cannot be removed by etching the metal plate 224, i.e., by theetching the metal plate 224 using an aqueous solution of ferric chlorideor an aqueous solution of cupric chloride normally used for etchingcopper. The conductive reactive layer 226 consequently remains on theceramic plate 206. If the conductive reactive layer 226 remains on theceramic plate 206, the circuit 210 may be short-circuited.

[0012] In order to solve the problem, a method has been suggested, inwhich any unnecessary brazing matter (etching residue) containing anitride layer of active metal, which remains after the circuit etchingwith the ferric chloride solution or the cupric chloride solution, isremoved by performing another acid treatment after the etching step.

[0013] For example, Japanese Patent No. 2594475 discloses a method forremoving the unnecessary brazing matter with hydrofluoric acid singly orwith mixed acid of inorganic acid and hydrofluoric acid. JapaneseLaid-Open Patent Publication No. 4-322491 discloses a method forremoving the unnecessary brazing matter with ammonium halide. JapaneseLaid-Open Patent Publication No. 5-13920 discloses a method for removingthe unnecessary brazing matter with inorganic acid and hydrogen peroxideafter treating the unnecessary brazing matter with hydrogen halide orammonium halide.

[0014] Japanese Laid-Open Patent Publication No. 7-235750 discloses amethod for removing the unnecessary brazing matter with a solutioncontaining fluorine compound and hydrogen peroxide but containing noinorganic acid. Japanese Laid-Open Patent Publication No. 10-154866discloses a method for removing the unnecessary brazing matter. Themethod comprises the steps of treating the unnecessary brazing matterwith ammonium fluoride and hydrogen peroxide and treating theunnecessary brazing matter with a solution of alkaline and hydrogenperoxide.

[0015] However, according to these techniques, it is necessary toconsider the safety of operation, because the hydrofluoric acid-basedsolution is used. Further, it is difficult to manage the steps, and itis impossible to completely remove the nitride layer of active metal.

[0016] It is assumed to use a technique for removing the unnecessarybrazing matter by the honing laser machining as a technique formechanically removing the unnecessary brazing matter (see, for example,Japanese Laid-Open Patent Publication No. 7-99380). However, the largeapparatus may be constructed and the production cost is also large.

SUMMARY OF THE INVENTION

[0017] It is therefore an object of the present invention to provide acircuit board which is excellent in both of appearance andcharacteristics and in which any etching residue such as a conductivereactive layer remaining on an insulating substrate can be removed withease, and a method for producing the same.

[0018] Another object of the present invention is to provide a circuitboard which controls the warpage of an entire joined unit so that acooling fin made of metal or the like may be strongly fixed, in additionto the above condition, and a method for producing the same.

[0019] The present invention provides a circuit board having a circuiton an insulating substrate. The circuit is formed by etching andsandblasting a plate of metal which is joined onto the insulatingsubstrate.

[0020] According to another aspect of the present invention, there isprovided a circuit board having a circuit on an insulating substrate,wherein the circuit is formed by sandblasting a plate of metal which isjoined onto the insulating substrate and which has a circuit pattern.

[0021] The present inventors have paid attention to the fact that thevelocity of removal differs between the metal material such as copper,aluminum, and silver brazing and the highly hard nonmetallic materialsuch as nitride and oxide when various materials are sandblasted. Thevelocity of removing the latter is several times to several tens timesthe velocity of removing the former.

[0022] The present inventors have found out the following fact. That is,only the nitride layer of the active metal, which has the fast velocityof removal in the sandblast, can be effectively removed with scarcelyaffecting the circuit of the metal material having the slow velocity ofremoval in the sandblast even when the sandblast is applied to theentire surface under the same condition for the metal material and theinsulating material if no residual matter of the metal material isallowed to remain between parts of the formed circuit, i.e., only thenitride layer of the active metal formed on the insulating substrate(insulating material) is allowed to remain after joining the metal platesuch as a copper plate to the insulating substrate with the active metalbrazing to form the circuit. Thus, the present invention has beencompleted.

[0023] The circuit is formed by pressing and etching the metal plate. Aplating layer may be stacked on the circuit of the metal.

[0024] In this case, it is preferable that a surface roughness of thecircuit of the metal or a surface roughness of the plating layer on thecircuit is not more than Ra=1. If the surface roughness exceeds Ra=1,for example, bonding wires hardly make tight contact in the wiring stepto be performed thereafter.

[0025] The circuit of the metal may be joined onto the insulatingsubstrate using a hard brazing member containing an active element. Inthis case, it is preferable that the hard brazing member has a thicknessof not more than 10 μm. The objective substance to be removed by thesandblast is at least a part of the conductive reactive layer which isgenerated by a reaction between the insulating substrate and the activeelement in the hard brazing member and which exists at a metal-removedportion of the circuit.

[0026] At lease one of elements belonging to any one of Group 2A, Group3A, Group 4A, Group 5A, and Group 4B in the periodic table can be usedas the active element contained in the hard brazing member.

[0027] It is also preferable that a heat spreader member or a heat sinkmember is joined to a lower portion of the insulating substrate. In thiscase, it is preferable that at least one selected from SiC, AlN, Si₃N₄,BeO, A1 ₂O₃, Be₂C, C, Cu, Cu alloy, Al, Al alloy, Ag, Ag alloy, and Siis used as a constitutive material for the heat spreader member or theheat sink member. Especially, it is preferable that the heat spreadermember is composed of a composite material in which an SiC base materialis impregnated with Cu or Cu alloy, or a composite material in which a Cbase material is impregnated with Cu or Cu alloy. The insulatingsubstrate may be composed of AlN or Si₃N₄.

[0028] When the heat spreader member is joined to the lower portion ofthe insulating substrate, a buffer plate of metal may be joined betweenthe insulating substrate and the heat spreader member using a hardbrazing member containing an active element, and a first joined unit maybe provided, which comprises the circuit of the metal, the insulatingsubstrate, the buffer plate, and the heat spreader member.

[0029] In this case, it is preferable that a ratio between a thicknessof the circuit of the metal and a total thickness of the buffer plateand the heat spreader member or the heat sink member is 1:0.5 to 1:3when a coefficient of thermal expansion of the insulating substrate inthe first joined unit is smaller than a coefficient of thermal expansionof a material used for the heat spreader member. Accordingly, the firstjoined unit is warped such that the lower surface of the metal plate isconvex-shaped toward the outside. When the heat spreader member isjoined or fixed to the lower surface of the metal plate, it is possibleto maintain the contact therebetween in a well-suited manner.

[0030] It is also preferable that a heat sink member is joined to alower surface of the heat spreader member using a hard brazing membercontaining an active element, or a metal plate for making joining to aheat sink member is joined using a hard brazing member containing anactive element, and a second joined unit is provided, which comprisesthe circuit of the metal, the insulating substrate, the buffer plate,and the heat spreader member and the heat sink member or the metal plateand the heat sink member. In this case, the heat sink member may have ashape of fin.

[0031] The warpage of the first joined unit or the second joined unitcan be controlled by appropriately changing the thickness of the bufferplate. It is preferable that the buffer plate has a thickness of 0.03 to0.5 mm.

[0032] The heat shock characteristics and the coefficient of thermalexpansion of the first joined unit or the second joined unit can becontrolled by appropriately changing the thickness of the insulatingsubstrate. Especially, it is preferable that the insulating substratehas a thickness of not more than 0.5 mm when the insulating substrate iscomposed of Si₃N₄. It is preferable that the first or second joined unithas a coefficient of thermal conductivity of not less than 200 W/m.

[0033] It is preferable to use the hard brazing member composed ofAg-Cu-In-Ti and materials having a melting point of not more than 700°C.

[0034] It is preferable that an amount of the active element, which iscontained in the hard brazing member to be used for joining at least thecircuit of the metal and the insulating substrate, is 0.05 to 2%.

[0035] It is preferable that an amount of the active element, which iscontained in the hard brazing member to be used for joining the bufferplate and the heat spreader member or the heat sink member, is 0.5 to10%.

[0036] It is preferable that a thickness of the hard brazing member tobe used for joining the heat spreader member or the heat sink member andthe buffer plate, or a thickness of the hard brazing member to be usedfor joining the heat spreader member or the heat sink member and themetal plate is not more than 25% of a thickness of the buffer plate.Accordingly, it is possible to enhance the peel strength of the firstjoined unit or the second joined unit.

[0037] It is preferable that a thickness of the hard brazing member tobe used for joining the insulating substrate and the circuit of themetal, or a thickness of the hard brazing member to be used for joiningthe insulating substrate and the buffer plate is not more than 10% of athickness of the buffer plate. For example, it is preferable that thethickness of the hard brazing member is not more than 10 μm.Accordingly, a large amount of the brazing member does not stick outduring the joining process. Further, the circuit is not polluted(alloyed). Consequently, the subsequent steps are simple for improvingthe yield and for reducing the production cost.

[0038] According to still another aspect of the present invention, thereis provided a method for producing a circuit board (first productionmethod), comprising a first step of joining a metal plate onto aninsulating substrate using a hard brazing member containing an activemetal; a second step of etching the metal plate to form a circuitpattern on the insulating substrate; and a third step of exposing theinsulating substrate by removing a conductive reactive layer exposedfrom at least a metal-removed portion of the circuit pattern to obtainthe circuit board having a circuit on the insulating substrate.

[0039] According to still another aspect of the present invention, thereis provided a method for producing a circuit board (second productionmethod), comprising a first step of forming a circuit pattern for ametal plate; a second step of joining the metal plate onto an insulatingsubstrate using a hard brazing member containing an active metal; and athird step of exposing the insulating substrate by removing a conductivereactive layer exposed from at least a metal-removed portion of thecircuit pattern to obtain the circuit board having a circuit on theinsulating substrate.

[0040] In the second production method, the first step may include atreatment of pressing the metal plate to form the circuit pattern on themetal plate. In this case, the method may further includes a treatmentof forming a bridge for connecting parts of the circuit and cutting thebridge after joining the metal plate onto the insulating substrate. Thebridge may be formed by half blanking or etching the metal plate.

[0041] The metal plate may be joined onto the insulating plate at atemperature of not less than a liquidus curve of the hard brazingmember. Alternatively, the metal plate may be joined onto the insulatingplate at a temperature of not less than a solidus curve and not morethan a liquidus curve of the hard brazing member. Even when any one ofthe methods is adopted, there is no influence on the heat shockcharacteristics and the coefficient of thermal conductivity.

[0042] In each of the production methods described above, for example,the conductive reactive layer, which remains at the metal-removedportion of the circuit of those on the insulating substrate, is removed.Therefore, it is possible to obtain the circuit board which is excellentin both of appearance and characteristics.

[0043] Especially, in the third step of the first and second productionmethods, the conductive reactive layer, which is exposed from themetal-removed portion of the circuit pattern, is removed by sandblastingan entire surface including the circuit pattern to expose the insulatingsubstrate. Accordingly, it is possible to easily remove any etchingresidue such as the conductive reactive layer remaining on theinsulating substrate. It is possible to obtain the circuit board whichis excellent in both of appearance and characteristics.

[0044] In the third step, the conductive reactive layer, which isexposed from the metal-removed portion of the circuit pattern, may beremoved by selectively performing a sandblast using a mask to expose theinsulating substrate. Alternatively, a plurality of masks may be used,and a treatment, in which the sandblast is selectively performed usingeach of the masks, and then the conductive reactive layer exposed fromwindows of the masks and exposed from the metal-removed portion of thecircuit pattern is removed, may be repeatedly performed with theplurality of masks.

[0045] When the mask is used, the surface of the circuit pattern is notscraped by the blasting abrasive grains. Therefore, it is possible tomaintain the surface of the circuit pattern to be a mirror-finishedsurface. For example, it is possible to adequately join bonding wires inthe wiring step to be performed thereafter.

[0046] The metal plate may be composed of Cu, Cu alloy, Al, or Al alloy.Further, a plating layer may be formed on the metal plate. In this case,for example, when a semiconductor device, which is mounted on thecircuit, is soldered, then the wettability of the solder layer isimproved, and the semiconductor device can be reliably mounted on thecircuit. It is preferable that the plating layer is an Ni plating layer.

[0047] In the first step of the first and second production methods, themetal plate is joined onto the insulating substrate using the hardbrazing member containing the active element. In this case, it ispossible for the active element to select at lease one of elementsbelonging to any one of Group 2A, Group 3A, Group 4A, Group 5A, andGroup 4B in the periodic table.

[0048] It is preferable that the sandblast in the third step isperformed on condition that at least the circuit pattern remains on theinsulating substrate at a stage at which the insulating substrate isexposed. When a plating layer is formed on the circuit, it is preferablethat the sandblast in the third step is performed on condition that theplating layer remains on the circuit pattern at a stage at which theinsulating substrate is exposed.

[0049] In the third step, the conductive reactive layer is generated bya reaction between the insulating substrate and the active element inthe hard brazing member. In this situation, it is necessary that a partof the conductive reactive layer, which corresponds to the metal-removedportion of the circuit pattern, is removed by the sandblast. In thiscase, it is preferable that the sandblast is performed on condition thatat least the circuit pattern remains on the insulating substrate at astage at which the part of the conductive reactive layer correspondingto the metal-removed portion of the circuit is removed. When a platinglayer is formed on the circuit pattern, it is preferable that thesandblast is performed on condition that the plating layer remains onthe circuit pattern at a stage at which the part of the conductivereactive layer corresponding to the metal-removed portion of the circuitpattern is removed.

[0050] It is preferable that grains, which are smaller than mesh #180,are used for the sandblast. It is preferable that the grains arecomposed of Al₂ 0 ₃ or SiC. Further, it is preferable that an airpressure is 0.1 MPa to 0.25 MPa in the sandblast.

[0051] In order not to allow any residual matter of the metal materialto remain between parts of the formed circuit pattern composed of, forexample, copper, it is preferable that an etching is performed with anaqueous solution of ferric chloride or an aqueous solution of cupricchloride to be normally used when copper is etched, and then a treatmentis performed with a solution to effectively etch components of thebrazing member.

[0052] It is possible for the joining process to use the hard brazingmember containing a major component of Ag and containing the activeelement or the brazing member containing a major component of Agtogether with a foil of the active metal. Therefore, it is preferable toperform a treatment with an aqueous solution of ferric nitrate in orderto etch Ag.

[0053] The waste of the brazing member should be thin. It is preferablethat the thickness thereof is not more than 10 μm, more preferably notmore than 5 μm by performing, for example, the joining process under anapplied pressure, because it is possible to omit the step of removingthe brazing member components.

[0054] When the pressure is applied during the joining process, it isenough to apply a stress which is sufficient to maintain the flatness ofthe joining objective during the melting of the brazing member andextrude the excessive brazing member to the outside. For preventing thedestruction of the insulating member, it is appropriate to perform thejoining process at a stress of 0.1 MPa to 20 MPa, and more preferably0.5 MPa to 10 MPa.

[0055] The velocity of removal of the brazing member component isapproximate to the velocity of removal of the copper circuit componentduring the blasting. However, with the above-mentioned thickness, thecopper circuit is suitably removed, even when the brazing member wasteremains after the etching with the aqueous solution of ferric chlorideor the aqueous solution of cupric chloride. It is possible to remove thebrazing member layer and the nitride layer of the active metal.

[0056] A plating layer may be stacked on the circuit of the metal plate.Also in this case, when the thickness of the plating layer is not lostby the blasting necessary to remove the residual brazing member wasteand the nitride layer of the active metal, it is possible to form thecopper circuit with the plating layer in accordance with the same orequivalent treatment.

[0057] When Ni plating is performed, the thickness is preferably notless than 2 μm and more preferably not less than 5 μm, considering theremaining brazing member.

[0058] Further, in the joining process, it is possible to use the hardbrazing member containing a major component of Cu and containing theactive element or the brazing member containing a major component of Cutogether with a foil of the active metal. In this case, it is possibleto perform an etching by only etching with the aqueous solution offerric chloride or the aqueous solution of cupric chloride withoutallowing the hard brazing member component to remain.

[0059] A heat spreader member or a heat sink member may be joined to alower portion of the insulating substrate. In this case, a buffer plateof metal may be joined to the lower surface of the insulating substrateusing a hard brazing member, and the heat spreader member or the heatsink member may be joined to the lower surface of the buffer plate usinga hard brazing member. Further, the metal plate may be joined to thelower surface of the heat spreader member or the heat sink member usinga hard brazing member.

[0060] When the buffer plate, the heat spreader member or the heat sinkmember, and the metal plate are joined to the lower portion of theinsulating substrate using the hard brazing member respectively, it isdesirable to reduce the amount of the brazing member in order tosuppress the pollution of the circuit caused by formation of alloy, whenAg-Cu-In-Ti is used as the hard brazing member. However, in such asituation, an absolute amount of necessary Ti is insufficient in somecases. In such a case, it is desirable that a Ti foil is mixed dependingon a required amount so that the Ti concentration is increased to effectthe joining. Accordingly, it is possible to improve the peel strength ascompared with a case in which the Ti foil is not mixed. The required Ticoncentration may depend on the material to be joined. However, it isdesirable that the concentration is not less than 0.05 mg/cm² in thecase of the joining of Cu and AlN or Si₃N₄ of the present invention, orthe concentration is not less than 1.5 mg/cm² in the case of the joiningof a composite material in which a base material of Cu and SiC isimpregnated with Cu or Cu alloy or a composite material in which a basematerial of C is impregnated with Cu or Cu alloy.

[0061] The above and other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich a preferred embodiment of the present invention is shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is a vertical sectional view illustrating a circuit boardaccording to a first embodiment and an electronic part using the circuitboard;

[0063]FIG. 2 is a magnified view illustrating a composite material ofSiC/Cu as an example of the constitutive material for a heat sinkmember;

[0064]FIG. 3 is a magnified view illustrating a composite material ofC/Cu as another example of the constitutive material for the heat sinkmember;

[0065]FIG. 4 is a magnified sectional view illustrating a part of acircuit and an insulating substrate of the circuit board according tothe first embodiment;

[0066]FIG. 5A illustrates a setting step, and FIG. 5B illustrates ajoining step;

[0067]FIG. 6A illustrates a resist-forming step, and FIG. 6B illustratesan etching step;

[0068]FIG. 7 illustrates a sandblast step;

[0069]FIG. 8 shows a table illustrating etching rates for variousmaterials in the sandblast;

[0070]FIG. 9 illustrates another example of the sandblast step;

[0071]FIG. 10A is a plan view illustrating a first mask, and FIG. 10B isa plan view illustrating a second mask;

[0072]FIG. 11A shows an example of a planar pattern of a circuit afterthe etching, FIG. 11B shows a planar pattern of the circuit afterperforming the sandblast using the first mask, and FIG. 11C shows aplanar pattern of the circuit after performing the sandblast using thesecond mask;

[0073]FIGS. 12A to 12E show steps illustrating an example of a methodbased on press forming in a second production method;

[0074]FIGS. 13A to 13E show steps illustrating another example of amethod based on press forming in the second production method;

[0075]FIGS. 14A to 14D show steps illustrating an example of a methodbased on etching in the second production method;

[0076]FIG. 15 shows a table illustrating results of a first illustrativeexperiment (heat cycle test for Comparative Example 1 and Examples 1 to3);

[0077]FIG. 16 shows characteristics illustrating results of Example 2 ina second illustrative experiment (illustrative experiment to observe thechange of the coefficient of thermal conductivity of the circuit boarddepending on the coefficient of thermal conductivity of the insulatingsubstrate);

[0078]FIG. 17 shows characteristics illustrating results of ComparativeExample 2 in the second illustrative experiment;

[0079]FIG. 18 shows characteristics illustrating results of ComparativeExample 3 in the second illustrative experiment;

[0080]FIG. 19 shows characteristics illustrating results of ComparativeExample 4 in the second illustrative experiment;

[0081]FIG. 20 shows a table illustrating results of a third illustrativeexperiment (illustrative experiment to observe the heat cycle, thecoefficient of thermal conductivity of the circuit board, and theinsulation performance of the insulating substrate for ComparativeExample 5 and Examples 3 to 6);

[0082]FIG. 21 shows characteristics illustrating results of a fourthillustrative experiment (illustrative experiment to observe the changeof the coefficient of thermal conductivity of the circuit boarddepending on the coefficient of thermal conductivity of the insulatingsubstrate itself);

[0083]FIG. 22 shows a table illustrating results of a fifth illustrativeexperiment (illustrative experiment to observe the change of the amountof warpage of the circuit board depending on the thickness of thecircuit);

[0084]FIG. 23 shows characteristics illustrating results of the fifthillustrative experiment;

[0085]FIG. 24 shows a table illustrating results of a sixth illustrativeexperiment (illustrative experiment to observe the change of the amountof warpage of the circuit board depending on the thickness of the bufferplate);

[0086]FIG. 25 shows characteristics illustrating results of the sixthillustrative experiment;

[0087]FIG. 26 shows characteristics illustrating results of a seventhillustrative experiment (illustrative experiment to observe the changeof the coefficient of thermal conductivity of the circuit board when thethickness of the buffer plate is changed);

[0088]FIG. 27 shows characteristics illustrating results of an eighthillustrative experiment (illustrative experiment to observe the changeof the amount of warpage of the circuit board depending on the thicknessof the metal plate);

[0089]FIG. 28 shows characteristics illustrating results of a ninthillustrative experiment (illustrative experiment to observe thedifference in heat cycle and coefficient of thermal conductivitydepending on the joining temperature for Examples 34 to 36);

[0090]FIG. 29 shows characteristics illustrating results of a tenthillustrative experiment (illustrative experiment to observe thedifference in coefficient of thermal conductivity of the circuit boarddepending on the residual thickness of each of first and second brazingmembers for Examples 37 to 40);

[0091]FIG. 30 shows a table illustrating results of an eleventhillustrative experiment (illustrative experiment to observe thedifference in pollution state of the circuit, peel strength, heat cycle,and coefficient of thermal conductivity depending on the thickness ofeach of first to fourth brazing members for Comparative Examples 8 to 10and Examples 41 to 43);

[0092]FIG. 31 shows a table illustrating results of a twelfthillustrative experiment (illustrative experiment to observe thedifference in peel strength, heat cycle, and coefficient of thermalconductivity depending on the amount of the active element in each offirst to fourth brazing members for Comparative Example 11 and Examples44 to 46);

[0093]FIG. 32 shows a table illustrating results of a thirteenthillustrative experiment (illustrative experiment to observe thedifference in tight contact performance of bonding wire with respect tothe circuit (or the plating layer) depending on the specular reflectionof the circuit (or the plating layer) for Comparative Example 12 andExamples 47 to 53);

[0094]FIG. 33 is a vertical sectional view illustrating a circuit boardaccording to a second embodiment and an electronic part based on the useof the circuit board;

[0095]FIG. 34 is a sectional view illustrating a circuit board having astructure in which a fin-shaped metal plate is joined to a lower surfaceof a heat sink member;

[0096]FIG. 35 is a sectional view illustrating a circuit board in whicha heat sink member itself is fin-shaped;

[0097]FIG. 36 is a vertical sectional view illustrating a conventionalelectronic part; and

[0098]FIG. 37 is a magnified sectional view illustrating a part of acircuit and an insulating substrate of the conventional circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0099] Illustrative embodiments of the circuit board and the method forproducing the same according to the present invention will be explainedbelow with reference to FIGS. 1 to 35.

[0100] At first, as shown in FIG. 1, an electronic part 12A having acircuit board 10A according to a first embodiment comprises asemiconductor device 16 which is mounted on the circuit board 10A with asolder layer 14 interposed therebetween, and a cooling fin 20 which isfixed to the lower surface of the circuit board 10A with a metal plate18 interposed therebetween.

[0101] The circuit board 10A according to the first embodiment comprisesa thermal conductive layer 24 which is provided on a heat spreadermember 22.

[0102] The thermal conductive layer 24 comprises a buffer plate (Cu orthe like) 28 made of metal which is joined onto the heat spreader member22 with a first brazing member 26 containing an active elementinterposed therebetween, an insulating substrate 32 which is joined ontothe buffer plate 28 with a second brazing member 30 containing an activeelement interposed therebetween, and a circuit 36 of a circuit-formingmetal plate which is joined onto the insulating substrate 32 with athird brazing member 34 interposed therebetween. The metal plate 18 (Cuor the like) is joined to the lower surface of the heat spreader member22 with a fourth brazing member 100 interposed therebetween.

[0103] In this embodiment, the circuit 36 is manufactured such that thecircuit-forming metal plate 40, which is composed of, for example, Cu orAl and which has an Ni plating layer 38 formed on the upper surface, issubjected to press working or etching treatment along a predeterminedcircuit pattern.

[0104] An AlN layer or an Si₃N₄ layer can be used for the insulatingsubstrate 32. When the AlN layer is used for the insulating substrate32, the coefficient of thermal expansion of the AlN layer isapproximately within a range of 3.0×10⁻⁶ to 1.0×10⁻⁵ /K, although thecoefficient of thermal expansion of the AlN layer changes depending onthe molar composition ratio of Al and N. Therefore, it is preferablethat the coefficient of thermal expansion of the heat spreader member 22is 3.0×10⁻⁶ to 1.0×10⁻⁵ /K, for the following reason. If the coefficientof thermal expansion of the insulating substrate 32 is 3.0×10⁻⁶ /K, andthe coefficient of thermal expansion of the heat spreader member 22exceeds 1.0×10⁻⁵ /K, then the heat spreader member 22 and the insulatingsubstrate 32 may be peeled off or exfoliated from each other, when thetemperature of the electronic part 12A in use is raised.

[0105] It is preferable that the molar composition ratio of Al to N inthe insulating substrate 32 is Al:N=0.8:1.2 to 1.2:0.8, for thefollowing reason. When such a molar composition ratio is adopted, theinsulating substrate 32 reliably exhibits a coefficient of thermalexpansion of 3.0 ×10⁻⁶ to 1.0×10⁻⁵ /K and a coefficient of thermalconductivity of not less than 150 W/mK.

[0106] It is preferable that the coefficient of thermal conductivity ofthe heat spreader member 22 is not less than 150 W/mK, for the followingreason. If the coefficient of thermal conductivity is less than 150W/mK, the heat, which is generated by the semiconductor device 16 as theelectronic part 12A is used, is transmitted at a slow speed to theoutside of the electronic part 12A. As a result, a poor effect isobtained to maintain a constant temperature of the electronic part 12A.

[0107] The constitutive material for the heat spreader member 22 is notspecifically limited provided that the coefficient of thermalconductivity and the coefficient of thermal expansion are within theranges described above. However, the constitutive material for the heatspreader member 22 is preferably exemplified by at least one selectedfrom the group consisting of SiC, AlN, Si₃N₄, BeO, Al₂ 0 ₃, Be₂C, C, Cu,Cu alloy, Al, Al alloy, Ag, Ag alloy, and Si. That is, the heat spreadermember 22 can be constructed with a simple substance selected from theabove, or a composite material composed of two or more of the above. Thecomposite material can be exemplified by an SiC/Cu composite material22A (see FIG. 2) and a C/Cu composite material 22B (see FIG. 3).

[0108] As shown in FIG. 2, the SiC/Cu composite material 22A is obtainedby impregnating open pores 52 of a porous sintered matter 50 composed ofSiC with melted Cu or Cu alloy 54, and then solidifying the Cu or Cualloy 54.

[0109] As shown in FIG. 3, the C/Cu composite material 22B is obtainedby impregnating open pores 62 of a porous sintered matter 60 obtained bypreliminarily sintering carbon or allotrope thereof to form a network,with melted Cu or Cu alloy 64, and then solidifying the Cu or Cu alloy64. For example, the C/Cu composite material 22B is a member asdisclosed in Japanese Patent Application No. 2000-80833.

[0110] When the heat spreader member 22 is composed of the compositematerial or the alloy as described above, the coefficient of thermalexpansion and the coefficient of thermal conductivity can be controlledto be within the ranges as described above (coefficient of thermalexpansion: 3.0×10⁻⁶ to 1.0×10⁻⁵ /K, coefficient of thermal conductivity:not less than 150 W/mK) by establishing the composition ratio of theconstitutive components.

[0111] It is preferable that each of the first to fourth brazing members26, 30, 34, 100 is a hard brazing member containing an active element.In this case, as for the active element, it is possible to use at leaseone of elements belonging to Group 2A in the periodic table including,for example, Mg, Sr, Ca, Ba, and Be, Group 3A including, for example,Ce, Group 4A including, for example, Ti, Zr, and Hf, Group 5A including,for example, Nb and Ta, and Group 4B including, for example, B and Si.In the first embodiment, a hard brazing member of Ag-Cu-Ti or a hardbrazing member of Ag-Cu-In-Ti or Cu-Al-Si-Ti is used for the first tothird brazing members 26, 30, 34. In this case, the active element isTi.

[0112] When the circuit board 10A according to the first embodiment isused, the heat sink member 20, which is composed of, for example, Al orCu, is fixed, for example, by means of screw fastening (not shown) tothe lower surface of the metal plate 18. As shown in the drawings, thecooling fin is generally provided as the heat sink member 20, which is amember for dissipating the heat by means of water cooling or aircooling.

[0113] As shown in a magnified view in FIG. 4, in the circuit board 10Aaccording to the first embodiment, the circuit 36 is formed by means ofthe following two types of methods. That is, in the first method, thecircuit 36 is formed by means of an etching treatment and a sandblasttreatment for the circuit-forming metal plate 40 which is joined ontothe insulating substrate 32 with the third brazing member 34 interposedtherebetween. In the second method, the circuit-forming metal plate 40,on which a circuit pattern is previously formed (indicating that metalportions other than the circuit 36 are not completely removed, andportions to be converted into the circuit 36 thereafter are depicted asa pattern), is joined onto the insulating substrate 32 with the thirdbrazing member 34 interposed therebetween, and then a sandblasttreatment is performed for the circuit-forming metal plate 40 to formthe circuit 36.

[0114] The circuit 36 includes a metal-removed portion 36 a which isconstructed such that the underlying insulating substrate 32 is exposed.When the insulating substrate 32 and the circuit 36 are joined, aconductive reactive layer 70 is generated between the insulatingsubstrate 32 and the circuit 36 as a result of a reaction between thethird brazing member 34 and the insulating substrate 32. Therefore, inFIG. 4 the reference numeral 34 to indicate the third brazing member isshown with parentheses.

[0115] Several methods for producing the circuit board 10A according tothe first embodiment will now be explained with reference to FIGS. 5A to14D.

[0116] At first, the first production method includes a setting stepshown in FIG. 5A, in which the metal plate 18, the fourth brazing member100, the heat spreader member 22, the first brazing member 26, thebuffer plate 28, the second brazing member 30, the insulating substrate32, the third brazing member 34, and the circuit-forming metal plate 40plated with the Ni plating layer 38 on the upper surface are placed(subjected to setting) in this order on a jig 72, and the components arefixed on the jig 72. The setting is performed, for example, in theatmospheric air.

[0117] Subsequently, in a joining step shown in FIG. 5B, the metal plate18, the fourth brazing member 100, the heat spreader member 22, thefirst brazing member 26, the buffer plate 28, the second brazing member30, the insulating substrate 32, the third brazing member 34, and thecircuit-forming metal plate 40, which are fixed on the jig 72, arepressurized vertically, for example, in vacuum of not more than 1.0×10⁻⁵Torr, while the components are joined to one another by raising/loweringthe temperature. As a result of the joining treatment, a joined unit isobtained, in which the circuit-forming metal plate 40, the insulatingsubstrate 32, the buffer plate 26, heat spreader member 22, and themetal plate are integrated into one unit.

[0118] The pressure is applied in the joining step preferably at a forceor stress of not less than 0.1 MPa and not more than 20 MPa, and morepreferably not less than 0.5 MPa and not more than 10 MPa for thefollowing reason. If the stress is not more than the value as describedabove, then the brazing member layer remains, and the brazing member maybe an obstacle upon removal. On the other hand, if the stress is notless than the value as described above, excessive load may be applied tothe insulating substrate 32. Especially, in the joining operation, forexample, as shown in FIG. 6A, the conductive reactive layer 70 (TiNlayer) is formed between the metal plate 40 and the insulating substrate32 by the reaction between the active metal (Ti in this case) of thethird brazing member 34 and the insulating substrate 32 (AlN or Si₃N₄).

[0119] After that, as shown in FIGS. 6A and 6B, an etching treatment isapplied to the circuit-forming metal plate 40 along a predeterminedcircuit pattern to form the circuit 36. Specifically, as shown in FIG.6A, a circuit-forming resist 80 is printed on the entire surface of thecircuit-forming metal plate 40. Only portions, which are not subjectedto the etching, are selectively cured for the resist 80. Subsequently,non-cured portions are removed to form windows 80 a (resist-formingstep). The portions, on which the resist 80 remains, are formed into thecircuit pattern.

[0120] After that, as shown in FIG. 6B, the Ni plating 38 as a portionof the Ni plating layer 38 formed on the upper surface of the metalplate 40, exposed from the window 80 a of the resist 80 and the metalplate 40 are subjected to an etching treatment with an aqueous solutionof ferric chloride or an aqueous solution of cupric chloride to form thecircuit 36. It is preferable that an etching treatment is thereafterperformed with an aqueous solution of ferric nitrate when any brazingmember containing a major component of Ag is used, in order tocompletely remove the brazing member remaining between parts of thecircuit 36 (etching treatment step).

[0121] After that, as shown in FIG. 7, the resist 80 on the circuit 36is removed, and then a sandblast treatment is applied to the entiresurface including the circuit 36 to remove the conductive reactive layer70 remaining at the metal-removed portion 36 a of the circuit 36.Accordingly, the underlying insulating substrate 32 is exposed from themetal-removed portion 36 a of the circuit 36.

[0122] In this case, it is preferable that the sandblast treatment isperformed under a condition in which the Ni plating layer 38 remains onthe circuit 36 at the stage at which the conductive reactive layer 70remaining at the metal-removed portion 36 a of the circuit 36 isremoved.

[0123] In this embodiment, the grains, which are finer than mesh #180and which are composed of Al₂ 0 ₃ or SiC, are used for the sandblasttreatment. The air pressure is 0.1 MPa to 0.25 MPa in the sandblasttreatment. The thickness of the Ni plating layer 38 is not less than 2μm. The diameter of the grain corresponding to mesh #180 can berepresented by the maximum diameter of the grain which passes throughmesh #180. In this case, the diameter is 85 μm. Therefore, it ispreferable to use grains having a grain diameter of not more than 85 μm.

[0124] The etching rate will now be specifically explained for a varietyof materials for the sandblast treatment. At first, the etching rate wasmeasured by using Al₂O₃ grains having a fineness of mesh #240 as grainsto be used for the sandblast treatment and using air pressures of 0.1MPa and 0.25 MPa. Results were obtained as shown in FIG. 8.

[0125] That is, when the air pressure was 0.25 MPa, then the etchingrate for AlN as a constitutive material for the insulating substrate 32was 13 μm/sec, and the etching rate for Si₃N₄ as a constitutive materialfor the insulating substrate 32 was 2.7 μm/sec. Similarly, the etchingrate for Cu as a constitutive material for the circuit 36 was 1.5μm/sec, the etching rate for Ni as a constitutive material for the Niplating layer 38 on the circuit 36 was 2 μm/sec, the etching rate forAg-Cu-Ti as a brazing member was 1.6 μm/sec, and the etching rate forTiN as a constitutive material for the conductive reactive layer 70 was10 μm/sec.

[0126] The thickness of the conductive reactive layer 70, which isgenerated between the metal plate 40 and the insulating substrate 32, isabout 2 to 5 μm at most. Therefore, when the Ni plating layer 38 on themetal plate 40 has a thickness of not less than 2 μm, then theconductive reactive layer 70, which remains at the metal-removed portion36 a of the circuit 36, can be reliably removed by means of thesandblast treatment for 1 second, and the Ni plating layer 38 is allowedto remain on the circuit 36. Considering the stability of operation, itis preferable that the thickness of the Ni plating layer 38 is not lessthan 5 μm.

[0127] Similarly, when the air pressure was 0.1 MPa, then the etchingrate for AlN as a constitutive material for the insulating substrate 32was 4.8 μm/sec, and the etching rate for Si₃N₄ as a constitutivematerial for the insulating substrate 32 was 1.4 μm/sec. Similarly, theetching rate for Cu as a constitutive material for the circuit 36 was0.8 μm/sec, the etching rate for Ni as a constitutive material for theNi plating layer 38 on the circuit 36 was 1.2 μm/sec, the etching ratefor Ag-Cu-Ti as a brazing member was 0.9 μm/sec, and the etching ratefor TiN as a constitutive material for the conductive reactive layer 70was 3 μm/sec.

[0128] Also in this case, the conductive reactive layer 70, whichremains at the metal-removed portion 36 a of the circuit 36, can bereliably removed by means of the sandblast treatment for about 2seconds, and the Ni plating layer 38 is allowed to remain on the circuit36.

[0129] Of course, as shown in FIG. 9, a mask 82 may be installed on thecircuit 36. The portion, which is exposed through the mask 82, i.e., theconductive reactive layer 70 remaining at the metal-removed portion 36 aof the circuit 36, may be removed by means of the sandblast treatment.

[0130] In this case, for example, as shown in FIGS. 10A and 10B, aplurality of masks 82A, 82B may be used. The sandblast treatment may beselectively performed by the masks 82A, 82B. The first mask 82A shown inFIG. 10A has, for example, windows 83 a formed along horizontal ruledlines. The second mask 82B shown in FIG. 10B has, for example, windows83 b formed along vertical ruled lines.

[0131] A pattern shown in FIG. 11A is assumed as a planar pattern of thecircuit 36 after the etching treatment. In FIG. 11A, an area indicatedby hatched lines represents an area in which the conductive reactivelayer 70 is exposed.

[0132] At first, the first mask 82A shown in FIG. 10A is used to performthe sandblast treatment. As a result of this treatment, portions of theexposed conductive reactive layer 70, which correspond, for example, tothe windows 83 a of the first mask 82A, are removed as shown in FIG.11B. During this process, the portions of the circuit 36 are preventedfrom collision of the grains (grains used in the sandblast treatment) bythe first mask 82A.

[0133] After that, the sandblast treatment is performed using the secondmask 82B shown in FIG. 10B. Accordingly, portions, which correspond tothe windows 83 b of the second mask 82B, are removed. That is, as shownin FIG. 11C, all of the conductive reactive layer 70, which has beenexposed from the metal-removed portion of the circuit 36, is removed.Consequently, the insulating substrate is exposed from the metal-removedportion of the circuit 36. Also in this case, the portions of thecircuit 36 are prevented from collision of the grains (grains used inthe sandblast treatment) by the second mask 82B.

[0134] It is preferable that each of the first and second masks 82A, 82Bis composed of a material such as stainless steel, preferably having athickness of 0.3 to 1.0 mm. Accordingly, each of the first and secondmasks 82A, 82B maintains the original planar shape even when thesandblast treatment is completed, making it possible to sufficientlyfunction as a mask for the sandblast treatment.

[0135] When the sandblast treatment is performed using the mask asdescribed above, it is possible to reliably remove the conductivereactive layer 70 remaining at the metal-removed portion 36 a of thecircuit 36. Further, the surface roughness can be maintained withoutexerting any influence of the sandblast treatment on the Ni platinglayer 38 on the circuit 36.

[0136] Subsequently, as shown in FIG. 1, the semiconductor device 16 ismounted on the circuit 36 using the solder layer 14. Thus, theelectronic part 12A is obtained. In the first embodiment, for example, acommercially available silicon-based IGBT (power semiconductor element)was joined with a low melting point solder. Further, although not shown,metal wires (bonding wires) were electrically connected to terminals ofthe semiconductor device 16 by wire bonding. Similarly, metal wires werealso connected to the circuit 36.

[0137] After that, the circuit board 10A, to which the semiconductordevice 16 was joined, was accommodated in a package. A commerciallyavailable silicone gel for potting was injected into the package, andcured. Accordingly, the electric insulation performance of the circuitboard 10A is enhanced, and the mechanical reliability thereof issecured.

[0138] Next, the second production method will be explained withreference to FIGS. 12A to 14D. The second production method is differentfrom the first production method described above in that acircuit-forming metal plate 40, on which a circuit pattern is previouslyformed, is joined onto the insulating substrate 32. The method forpreviously forming the circuit pattern on the circuit-forming metalplate 40 includes a method based on press forming and a method based onetching. In FIGS. 12A to 14D, the plating layer 38 formed on thecircuit-forming metal plate 40 is omitted from the illustration.

[0139] At first, the method based on the press working will beexplained. A circuit-forming metal plate 40 is prepared as shown in FIG.12A. After that, as shown in FIG. 12B, half blanking machining isperformed for the circuit-forming metal plate 40 so that a circuitpattern 102 is formed, and arc-shaped bridges 106, which connectportions 104 to be converted into the circuit thereafter, are formed.

[0140] In this procedure, as shown in FIG. 12B, the brazing member 34may be applied to the lower surfaces of the portions of thecircuit-forming metal plate to be converted into the circuit thereafter.The brazing member may be applied as follows. That is, a paste ofbrazing member may be applied. Alternatively, a plate-shaped brazingmember or a brazing member in powder form may be applied or stuck withan adhesive.

[0141] After that, as shown in FIG. 12C (see FIG. 5A), the metal plate18, the fourth brazing member 100, the heat spreader member 22, thefirst brazing member 26, the buffer plate 28, the second brazing member30, the insulating substrate 32, the third brazing member 34, and thecircuit-forming metal plate 40 are placed (subjected to setting) in thisorder on the jig 72, and the components are fixed on the jig.

[0142] In this procedure, when the brazing member 34 is already appliedto the lower surfaces of the portions to be converted into the circuitthereafter as shown in FIG. 12B, it is unnecessary to newly prepare thethird brazing member 34 as shown in FIG. 12C. The circuit-forming metalplate 40, in which the brazing member 34 has been applied to the lowersurfaces, may be directly placed on the insulating substrate 32.

[0143]FIG. 12C shows a state in which the circuit-forming metal plate40, in which the brazing member 34 is not applied to the lower surfacesof the portions to be converted into the circuit thereafter, is placedand fixed on the insulating substrate 32 while the third brazing member34 is disposed between the insulating substrate 32 and thecircuit-forming metal plate 40.

[0144] After that, a jig 110 (protecting jig) for protecting the bridges106 is placed on the circuit-forming metal plate 40. The protecting jig110 has, for example, a shape of rectangular parallelepiped or cubewhich has an areal size of such an extent that the circuit-forming metalplate 40 is covered therewith in plan view. The protecting jig 110 hasrecesses 112 which is open downwardly at portions corresponding to thecircular arc-shaped bridges 106. Further, the protecting jig 110 isconfigured so that portions 114 other than the recesses 112 contact withthe portions 104 to be converted into the circuit thereafter.

[0145] After that, the metal plate 18, the fourth brazing member 100,the heat spreader member 22, the first brazing member 26, the bufferplate 28, the second brazing member 30, the insulating substrate 32, thethird brazing member 34, and the circuit-forming metal plate 40, whichare fixed on the jig 72 (see FIG. 5), are pressurized vertically, forexample, in vacuum of not more than 1.0×10⁻⁵ Torr, while the componentsare joined to one another by raising/lowering the temperature. Duringthis process, the circuit-forming metal plate 40 is covered with theprotecting jig 110. Therefore, the bridges 106 are neither deformed norcut when the pressure is applied. Thus, it is possible to reliably maketight contact of the circuit-forming metal plate 40 with the insulatingsubstrate 32, and it is possible to avoid any occurrence of positionaldiscrepancy for each of the portions of the circuit-forming metal plate40 to be converted into the circuit thereafter.

[0146] After that, the protecting jig 110 is removed as shown in FIG.12D, and then the bridges 106 are cut and removed, for example, withnippers as shown in FIG. 12E. Thus, the circuit 36 of thecircuit-forming metal plate 40 is formed on the insulating substrate 32.In order to achieve the object as described above, it is sufficient thatthe bridges 106 are provided at necessary sites at which the respectiveportions of the circuit-forming metal plate 40 to be converted into thecircuit thereafter do not cause any positional discrepancy. All of thesites other than the respective portions to be converted into thecircuit thereafter do not necessarily have the shape of bridge.

[0147] After that, in the same manner as in the first production methoddescribed above, the sandblast treatment is performed for the entiresurface including the circuit 36 or using the mask 82. Thus, theconductive reactive layer 70, which is exposed from the metal-removedportion 36 a of the circuit 36, is removed.

[0148] Next, explanation will be made with reference to FIGS. 13A to 14Dfor several modified embodiments of the second production methoddescribed above.

[0149] At first, in the production method according to the firstmodified embodiment, when the circuit is formed by press working for thecircuit-forming metal plate 40 as shown in FIG. 13B after preparing thecircuit-forming metal plate 40 as shown in FIG. 13A, half blankingmachining is firstly performed for the circuit-forming metal plate 40.As a result of the half blanking machining, a circuit pattern 102 isformed, and bridges 106, which connect portions 104 to be converted intothe circuit thereafter, are formed. In an example shown in FIG. 13B, thebridges 106 are lifted upwardly as compared with the portions 104 to beconverted into the circuit thereafter. After that, an adhesive isapplied to the lower surfaces of the portions to be converted into thecircuit thereafter, of the circuit-forming metal plate.

[0150] Subsequently, as shown in FIG. 13C (see FIG. 5A), the metal plate18, the fourth brazing member 100, the heat spreader member 22, thefirst brazing member 26, the buffer plate 28, the second brazing member30, the insulating substrate 32, the third brazing member 34, and thecircuit-forming metal plate 40 plated with the Ni plating layer 38 onthe upper surface are placed (subjected to setting) in this order on thejig 72, and the components are fixed on the jig 72. In this procedure,the lower surfaces of the portions 104 to be converted into the circuitthereafter, of the circuit-forming metal plate 40 are adhered onto thethird brazing member 34. FIG. 13C shows a state in which thecircuit-forming metal plate 40 is placed and fixed on the insulatingsubstrate 32 with the third brazing member 34 interposed therebetween.

[0151] After that, as shown in FIG. 13D (see FIG. 5B), thecircuit-forming metal plate 40 is pressed vertically, for example, witha punch (not shown) having a flat lower surface. During this process,the bridges 106 are moved downwardly by being pressed by the punch.Accordingly, the bridges 106 are sheared. The sheared bridges 106 aredepressed by being pressed by the punch to the positions at which theyare sheared. However, even when the sheared bridges 106 are depresseduntil they are completely contacted with the brazing member 34 as shownin FIG. 13D, they are not adhered.

[0152] After that, as shown in FIG. 13E, the bridges 106, which havebeen placed on the third brazing member 34, are discharged to theoutside. The discharge may be effected by performing, for example, amethod in which a tape or the like applied with an adhesive is pressedagainst the bridges to pull out the bridges, and a method in which thebridges are mechanically pinched to pull out the bridges. Accordingly,the circuit 36 of the circuit-forming metal plate is formed on theinsulating substrate 32.

[0153] After that, as shown in FIG. 5B, the metal plate 18, the fourthbrazing member 100, the heat spreader member 22, the first brazingmember 26, the buffer plate 28, the second brazing member 30, theinsulating substrate 32, the third brazing member 34, and thecircuit-forming metal plate 40, which are fixed on the jig 72, arepressurized vertically, for example, in vacuum of not more than 1.0×10⁻⁵Torr, while the components are joined to one another by raising/loweringthe temperature.

[0154] After that, in the same manner as in the first production methoddescribed above, the sandblast treatment is performed for the entiresurface including the circuit 36 or using the mask 82. Thus, theconductive reactive layer 70, which is exposed from the metal-removedportion 36 a of the circuit 36, is removed.

[0155] Next, in the production method according to the second modifiedembodiment, a circuit-forming metal plate 40 is prepared as shown inFIG. 14A. After that, as shown in FIG. 14B, portions other than portions104 to be converted into the circuit thereafter of the circuit-formingmetal plate 40 are subjected to etching so that the thickness of theportions is thinned. At this stage, a circuit pattern 102 based on theetching treatment is formed on the circuit-forming metal plate 40.

[0156] After that, as shown in FIG. 14C (see FIG. 5A), the metal plate18, the fourth brazing member 100, the heat spreader member 22, thefirst brazing member 26, the buffer plate 28, the second brazing member30, the insulating substrate 32, the third brazing member 34, and thecircuit-forming metal plate 40 are placed (subjected to setting) in thisorder on the jig 72, and the components are fixed on the jig 72. In thisprocess, the circuit-forming metal plate 40 is placed and fixed whilethe surface, which is irregular or convex/concave as a result of theetching, is opposed to the third brazing member 34. Accordingly, theportions having the thin thickness of the circuit-forming metal plate 40function as bridges 106 to connect portions of the circuit.

[0157] After that, the metal plate 18, the fourth brazing member 100,the heat spreader member 22, the first brazing member 26, the bufferplate 28, the second brazing member 30, the insulating substrate 32, thethird brazing member 34, and the circuit-forming metal plate 40, whichare fixed on the jig 72 (see FIG. 5), are pressurized vertically, forexample, in vacuum of not more than 1.0×10⁻⁵ Torr, while the componentsare joined to one another by raising/lowering the temperature.

[0158] After that, the bridges 106 are cut and removed, for example,with nippers as shown in FIG. 14D. Thus, the circuit 36 of thecircuit-forming metal plate 40 is formed on the insulating substrate 32.In this procedure, it is sufficient that the bridges 106 are provided atnecessary sites at which the respective portions of the circuit-formingmetal plate 40 to be converted into the circuit thereafter do not causeany positional discrepancy. All of the sites other than the respectiveportions to be converted into the circuit thereafter do not necessarilyhave the shape of bridge, in the same manner as in the precedingembodiment.

[0159] After that, in the same manner as in the production methoddescribed above, the sandblast treatment is performed for the entiresurface including the circuit 36 or using the mask 82. Thus, theconductive reactive layer 70, which is exposed from the metal-removedportion 36 a of the circuit 36, is removed.

[0160] Preferred specified embodiments of the circuit board 10Aaccording to the first embodiment will now be explained below.

[0161] At first, as shown in FIG. 1, it is preferable that the bufferplate 28 is provided between the insulating substrate 32 and the heatspreader member 22. Owing to the buffer plate 28, it is possible toimprove the heat cycle characteristics of the circuit board 10A.

[0162] A first illustrative experiment will now be described. In thefirst illustrative experiment, the heat cycle test was performed forComparative Example 1 and Examples 1 to 3. In the Comparative Example 1,the buffer plate 28 is omitted from the circuit board 10A according tothe first embodiment. The Examples 1 to 3 are constructed inapproximately the same manner as the circuit board 10A according to thefirst embodiment. Specified joining structures are shown in FIG. 15.

[0163] The thermal shock condition for one cycle in the heat cycle testis such that a temperature of 65° C. is applied for 15 minutes, and thena temperature of 150° C. is applied for 15 minutes (in accordance withthe test condition of MIL-STD-833C 1010.6C in MIL Standards of USA). Inthe first illustrative experiment, the heat cycle tests of 500 cyclesand 3000 cycles were performed for the Comparative Example 1 and theExamples 1 to 3 to observe states of exfoliation of the respectivejoining structures.

[0164] Experimental results are shown in FIG. 15. It was revealed thatany exfoliation already occurred between the insulating substrate 32 andthe heat spreader member 22 at a stage of 500 cycles in the ComparativeExample 1, but no exfoliation occurred even after 3000 cycles in theExamples 1 to 3.

[0165] It is preferable that the thickness of the buffer plate 28 is notless than 0.03 mm, for the following reason. If the thickness is lessthan 0.03 mm, no function as the buffer plate 28 as described above maynot be exhibited.

[0166] Further, as for the insulating substrate 32, it is preferablethat the thickness of the insulating substrate 32 is not more than 0.3mm when the insulating substrate 32 is composed of Si₃N₄. Accordingly,even when the coefficient of thermal conductivity of the insulatingsubstrate 32 itself is 40 W/mK which is low, the coefficient of thermalconductivity of the circuit board 10A can be made to be not less than200 W/mK when the circuit board 10A is prepared.

[0167] Second and third illustrative experiments will now be described.In the second illustrative experiment, observation was made for thechange of the coefficient of thermal conductivity of the circuit board10A depending on the coefficient of thermal conductivity (40 W/mK, 60W/mK, 80 W/mK, 100 W/mK) of the insulating substrate 32 (Si₃N₄) forComparative Example 2 to 4 and Example 4. The thickness of theinsulating substrate 32 was 0.3 mm in the Example 2, the thickness ofthe insulating substrate 32 was 0.6 mm in the Comparative Example 2, thethickness of the insulating substrate 32 was 0.9 mm in the ComparativeExample 3, and the thickness of the insulating substrate 32 was 1.2 mmin the Comparative Example 4. Experimental results are shown in FIGS. 16to 19.

[0168] As for FIGS. 16 to 19, FIG. 16 shows the result of the Example 4,FIG. 17 shows the result of the Comparative Example 2, FIG. 18 shows theresult of the Comparative Example 3, and FIG. 19 shows the result of theComparative Example 4.

[0169] According to the results, it is appreciated that the coefficientof heat transfer, which is obtained when the circuit board 10A isprepared, is high, i.e., 221.5 W/mK in the Example 2 even when thecoefficient of heat transfer of the insulating substrate 40 itself islow, i.e., 40 W/mK.

[0170] On the other hand, in third illustrative experiment, observationwas made for the heat cycle, the coefficient of thermal conductivity ofthe circuit board 10A, and the insulation performance of the insulatingsubstrate 32 for Comparative Example 5 and Examples 5 to 8. Any one ofthe insulating substrates 32 used had a coefficient of thermalconductivity of 100 W/mK.

[0171] In the Comparative Example 5, a structure was provided, in whichthe thickness of the insulating substrate 32 was 0.635 mm in the circuitboard 10A according to the first embodiment. In the Examples 5 to 8, thethickness of the insulating substrate 32 was 0.3 mm, 0.15 mm, 0.10 mm,and 0.07 mm respectively in the circuit board 10A according to the firstembodiment. Specified joining structures and experimental results areshown in FIG. 20.

[0172] No exfoliation was observed in the heat cycle test for 3000cycles in all of the Comparative Example 5 and the Examples 5 to 8.Further, the insulation performance was satisfactory. As for thecoefficient of thermal conductivity, it is appreciated that the Examples5 to 8 exhibited greatly high values, i.e., 290 W/mK, 308 W/mK, 325W/mK, 340 W/mK, as compared with 250 W/mK in the Comparative Example 5.

[0173] Especially, it is appreciated that the coefficient of heattransfer of the circuit board 10A is scarcely changed when the thicknessof the insulating substrate (Si₃N₄) is 0.1 mm, according to experimentalresults shown in FIG. 21 (fourth illustrative experiment). In the fourthillustrative experiment, the change of the coefficient of heat transferof the circuit board 10A was observed when the thickness of the circuit36 was 0.3 mm, the thickness of the buffer plate 28 was 0.25 mm, thethickness of the heat spreader member 22 was 3.0 mm and the coefficientof thermal conductivity of the insulating substrate 32 itself was 40W/mK (Example 9), 60 W/mK (Example 10), 80 W/mK (Example 11), and 100W/mK (Example 12).

[0174] Further, it is preferable for the circuit board 10A according tothe first embodiment that the ratio between the thickness of the circuit36 and the total thickness of the buffer plate 28, the heat spreadermember 22, and the metal plate 18 is 1:0.5 to 1:3. Accordingly, thelower surface of the metal plate 18 is warped so that the lower surfacehas a convex shape toward the outside. For example, when the heat sinkmember 20 is joined or fixed to the lower surface of the metal plate 18,it is possible to enhance the tight contact performance.

[0175] Explanation will now be made for illustrative experiments (fifthto eighth illustrative experiments) in relation to the warpage of thecircuit board 10A depending on the thicknesses of the circuit 36, thebuffer plate 28, and the metal plate 18.

[0176] At first, in the fifth illustrative experiment, observation wasmade for the change of the amount of warpage of the circuit board 10Adepending on the thickness of the circuit 36. The amount of warpage ofthe circuit board 10A was measured when the thickness of the insulatingsubstrate 32 (Si₃N₄) was 0.3 mm, the thickness of the buffer plate 28was 0.3 mm, the thickness of the heat spreader member 22 was 3.0 mm, thethickness of the metal plate 18 was 0.3 mm, and the thickness of thecircuit 36 was 0.3 mm (Comparative Example 6), 1.0 mm (ComparativeExample 7), 3.0 mm (Example 13), and 10.0 mm (Example 14). A laserconfocal displacement meter (Model LT-8110) produced by Keyence was usedas a measuring apparatus.

[0177] Experimental results are shown in FIGS. 22 and 23. In FIG. 23,the plot of the Comparative Example 6 is indicated by an open circle,the plot of the Comparative Example 7 is indicated by an open triangle,the plot of the Example 13 is indicated by a solid circle, and the plotof the Example 14 is indicated by a solid triangle.

[0178] According to the results, the warpage is formed in both of theComparative Examples 6 and 7 such that the front surface of the circuit36 is convex (lower surface of the metal plate 18 is concave). Incontrast, the warpage is formed in both of Examples 13 and 14 such thatthe front surface of the circuit 36 is concave (lower surface of themetal plate 18 is convex). Further, as indicated by a virtual line m(straight line depicted by approximation in accordance with the leastsquare method for the plots of the Comparative Examples 6, 7 and theExamples 13, 14) shown in FIG. 23, it is understood that the amount ofwarpage can be controlled by changing the thickness of the circuit 36.It is also possible to make the amount of warpage to be zero bycontrolling the thickness of the circuit 36.

[0179] Next, in the sixth illustrative experiment, observation was madefor the change of the amount of warpage of the circuit board 10Adepending on the thickness of the buffer plate 28. In the sixthillustrative experiment, the thickness of the circuit 36 was 0.3 mm, thethickness of the insulating substrate 32 was 0.3 mm, the thickness ofthe heat spreader member 22 was 3.0 mm, the thickness of the metal plate18 was 0 mm (formation of the metal plate 18 was omitted), and thejoining pressure was 2.5 kgf/cm². The amount of warpage of the circuitboard 10A was measured while changing the type of the insulatingsubstrate 32 and the thickness of the buffer plate 28 for Examples 15 to26. Experimental results are shown in FIGS. 24 and 25.

[0180] According to the results, in Examples 15 and 16 in which AlN isused for the insulating substrate 32, the warpage is formed such thatthe front surface of the circuit 36 is concave (lower surface of theheat spreader member 22 is convex). On the other hand, in Examples 17 to26 in which Si₃N₄ is used for the insulating substrate 32, the warpageis formed such that the front surface of the circuit 36 is convex (lowersurface of the heat spreader member 22 is concave).

[0181] Especially, FIG. 25 shows a straight line n1 depicted byapproximation in accordance with the least square method for the plotsof the Examples 15 and 16 by using AlN for the insulating substrate 32,and a straight line n2 depicted by approximation in accordance with theleast square method for the plots of the Examples 17 to 26.

[0182] According to the results of the Examples 15 and 16, as thethickness of the buffer plate 28 is increased, the amount of warpage ofthe concave front surface of the circuit 36 is gradually decreased. Onthe other hand, according to the results of the Examples 17 to 26, asthe thickness of the buffer plate 28 is increased, the amount of warpageof the convex front surface of the circuit 36 is gradually increased.

[0183] According to this fact, it is appreciated that the amount ofwarpage of the circuit board 10A can be controlled by changing thethickness of the buffer plate 28. However, if the thickness of thebuffer plate 28 exceeds a thickness of 0.5 mm, when AlN is used for theinsulating substrate 32, then the amount of warpage of the concave frontsurface of the circuit 36 of the circuit board 10A almost disappears. Itmay be difficult to allow the cooling fin to make tight contact with thelower surface of the metal plate 18. In view of the above, it ispreferable that the thickness of the buffer plate 28 is 0.03 to 0.5 mm.

[0184] The change of the coefficient of heat transfer of the circuitboard 10A was investigated when the thickness of the buffer plate 28 waschanged. According to experimental results shown in FIG. 26 (seventhillustrative experiment), the coefficient of heat transfer of thecircuit board 10A is scarcely changed. In the seventh illustrativeexperiment, the change of the coefficient of heat transfer of thecircuit board 10A was observed when the coefficient of thermalconductivity of the insulating substrate 32 (Si₃N₄) itself was 90 W/mK,and the thickness of the buffer plate 28 was 0.1 mm (Example 27), 0.3 mm(Example 28), 0.6 mm (Example 29), and 0.9 mm (Example 30). Thethickness of the circuit 36 was 0.3 mm, the thickness of the insulatingsubstrate 32 was 0.3 mm, and the thickness of the heat spreader member22 was 3.0 mm.

[0185] Next, in the eighth illustrative experiment, observation was madefor the change of the amount of warpage of the circuit board 10Adepending on the thickness of the metal plate 18. In the eighthillustrative experiment, the thickness of the circuit 36 was 0.3 mm, thethickness of the buffer plate 28 was 0.3 mm, and the thickness of theheat spreader member 22 was 3.0 mm. The amount of warpage of the circuitboard 10A was measured by changing the thickness of the metal plate 18for Examples 31 to 33. In Example 31, the thickness of the insulatingsubstrate 32 (Si₃N₄) was 0.3 mm. In Example 32, the thickness of theinsulating substrate 32 (AlN) was 0.3 mm. In Example 33, the thicknessof the insulating substrate 32 (AlN) was 0.5 mm.

[0186] Experimental results are shown in FIG. 27. In FIG. 27, Example 31is depicted by solid line D31, Example 32 is depicted by solid line D32,and Example 33 is depicted by solid line D33.

[0187] In Example 31, the warpage is formed such that the front surfaceof the circuit 36 is convex (lower surface of the metal plate 18 isconcave) over a range of the thickness of the metal plate 18 of 0 mm to0.5 mm. As the thickness of the metal plate 18 is increased, the amountof warpage of the convex front surface of the circuit 36 is alsogradually increased.

[0188] In Example 32, the warpage is formed such that the front surfaceof the circuit 36 is concave (lower surface of the metal plate 18 isconvex) over a range of the thickness of the metal plate 18 of 0 mm to0.12 mm. As the thickness of the metal plate 18 is increased, the amountof warpage of the concave front surface of the circuit 36 is graduallydecreased. Further, in Example 32, the warpage is formed such that thefront surface of the circuit 36 is convex (lower surface of the metalplate 18 is concave) over a range of the thickness of the metal plate 18of 0.12 mm to 0.5 mm. As the thickness of the metal plate 18 isincreased, the amount of warpage of the convex front surface of thecircuit 36 is also gradually increased.

[0189] In Example 33, the warpage is formed such that the front surfaceof the circuit 36 is concave (lower surface of the metal plate 18 isconvex) over a range of the thickness of the metal plate 18 of 0 mm to0.19 mm. As the thickness of the metal plate 18 is increased, the amountof warpage of the concave front surface of the circuit 36 is graduallydecreased. Further, in Example 33, the warpage is formed such that thefront surface of the circuit 36 is convex (lower surface of the metalplate 18 is concave) over a range of the thickness of the metal plate 18of 0.19 mm to 0.5 mm. As the thickness of the metal plate 18 isincreased, the amount of warpage of the convex front surface of thecircuit 36 is also gradually increased.

[0190] The amount of warpage of the circuit board 10A can be controlledby changing the thickness of the metal plate 18.

[0191] In the circuit board 10A according to the first embodiment, it ispreferable that a hard brazing member having a composition ofAg-Cu-In-Ti is used for the first to fourth brazing members 26, 30, 34,100. It is preferable that the amount of the active element (Ti)contained in each of the second and third brazing members 30, 34 is 0.05to 2%. It is preferable that the amount of the active element (Ti)contained in each of the first and fourth brazing members 26, 100 is 0.5to 10%.

[0192] It is preferable that the thickness of each of the second andthird brazing members 30, 34 is not more than 10% of the thickness ofthe buffer plate 28. Specifically, it is preferable that the thicknessof each of the second and third brazing members 30, 34 is not more than10 μm. On the other hand, it is preferable that the thickness of each ofthe first and fourth brazing members 26, 100 is not more than 25% of thethickness of the buffer plate 28.

[0193] Next, explanation will be made for illustrative experiments(ninth to eleventh illustrative experiments) in relation to the first tofourth brazing members 26, 30, 34, 100.

[0194] At first, in the ninth illustrative experiment, observation wasmade for the difference in heat cycle and coefficient of thermalconductivity depending on the joining temperature for Examples 34 to 36.In all of Examples 34 to 36, a brazing member having the followingcomposition and properties was used for the first to fourth brazingmembers 26, 30, 34, 100, in which the thickness was 50 μm:

[0195] Composition: Ag(59)-Cu(27.25)-In(12.5)-Ti(1.25);

[0196] Melting point (liquid phase): 715° C.;

[0197] Melting point (solid phase): 605° C.;

[0198] Thermal conductivity: 70 W/mK;

[0199] Specific gravity: 9.7 g/cm³.

[0200] In Examples 34 and 35, the joining temperature and the joiningtime were 680° C. (temperature between the solid phase and the liquidphase) and 10 minutes respectively. In Example 36, the joiningtemperature and the joining time were 780° C. (temperature not less thanthe liquid phase) and 10 minutes respectively.

[0201] Experimental results are shown in FIG. 28. As appreciated fromthe experimental results, even when the joining temperature is thetemperature between the solid phase and the liquid phase, the results ofthe heat cycle test and the coefficient of thermal conductivity arescarcely changed as compared with the case in which the joiningtemperature is the temperature of not less than the liquid phase. Inother words, even when the first to fourth brazing members 26, 30, 34,100 are not converted into the liquid form, then the circuit 36, theinsulating substrate 32, the buffer plate 28, the heat spreader member22, and the metal plate 18 can be tightly joined, and the coefficient ofthermal conductivity of the circuit board 10A is satisfactory.

[0202] When the joining temperature is the temperature between the solidphase and the liquid phase of the brazing member, the brazing memberremains in the circuit board 10A. However, according to the followingtenth illustrative experiment, the coefficient of thermal conductivityscarcely differs even when the thickness of the brazing member remainingin the circuit board 10A is changed. In the tenth illustrativeexperiment, observation was made for the difference in coefficient ofthermal conductivity of the circuit board 10A depending on the residualthickness of each of the first and second brazing members 26, 30 forExamples 37 to 40. In Example 37, the residual thickness of each of thefirst and second brazing members 26, 30 was 150 μm. In Example 38, theresidual thickness was 50 μm. In Example 39, the residual thickness was10 μm. In Example 40, the residual thickness was 1 μm. The coefficientof thermal conductivity of the insulating substrate 32 (Si₃N₄) was 90W/mK. Experimental results are shown in FIG. 29.

[0203] According to the experimental results, the coefficient of thermalconductivity of the circuit board 10A is scarcely changed even when theresidual thickness of each of the first and second brazing members 26,30 is changed within a range of 1 μm to 150 μm.

[0204] Next, in the eleventh illustrative experiment, observation wasmade for the difference in pollution state, peel strength, heat cycle,and coefficient of thermal conductivity of the circuit 36 depending onthe thickness of each of the first to fourth brazing members 26, 30, 34,100 for Comparative Examples 8 to 10 and Examples 41 to 43. Experimentalresults are shown in FIG. 30. In FIG. 30, the “thickness of brazingmember (1)” refers to the thickness of each of the second and thirdbrazing members 30, 34, and the “thickness of brazing member (2)” refersto the thickness of each of the first and fourth brazing members 26,100.

[0205] The composition of each of the first to fourth brazing members26, 30, 34, 100 used in the experiment wasAg(59)-Cu(27.25)-In(12.5)-Ti(1.25). The joining temperature was 780° C.,and the joining time was 10 minutes.

[0206] The thickness of the circuit 36 was 0.3 mm, the thickness of theinsulating substrate 32 (Si₃N₄) was 0.3 mm, the thickness of the bufferplate 28 was 0.3 mm, the thickness of the heat spreader member 22 was2.7 mm, and the thickness of the metal plate 18 was 0.3 mm.

[0207] According to the experimental results, when the thickness of eachof the second and third brazing members 30, 34 is not less than 30 μm,unnecessary portions of the second and third brazing members 30, 34outflow to the outside to pollute the circuit 36 when the circuit 36,the insulating substrate 32, the buffer plate 28, the heat spreadermember 22, and the metal plate 18 are joined as shown in ComparativeExamples 8 and 9 and Example 41. Especially, when the thickness of eachof the second and third brazing members 30, 34 is 50 μm as inComparative Examples 8 and 9, unnecessary portions of the brazingmembers 30, 34 stick out in a large amount when the components arejoined, resulting in an extreme pollution state of the circuit 36.

[0208] On the other hand, as shown in Examples 42, 43 and ComparativeExample 10, the circuit 36 is scarcely polluted when the thickness ofeach of the second and third brazing members 30, 34 is 10 μm.

[0209] Further, as in Comparative Example 8 and Example 43, when thethickness of each of the first and fourth brazing members 26, 100 was 50μm, the peel strength of the circuit board 10A was 85 kgf/cm² and 90kgf/cm². As in Examples 41 and 42, when the thickness of each of thefirst and fourth brazing members 26, 100 was 30 μm, the peel strength ofthe circuit board 10A was 52 kgf/cm² and 55 kgf/cm². Further, as inComparative Examples 9 and 10, when the thickness of each of the firstand fourth brazing members 26, 100 was 10 μm, the peel strength of thecircuit board 10A was 32 kgf /cm² and 30 kgf/cm².

[0210] The peel strength of the circuit board 10A is low when thethickness of each of the first and fourth brazing members 26, 100 isthin. No exfoliation was observed for any one of Comparative Examples 8to 10 and Examples 41 to 43 in the heat cycle test of 500 cycles. As forthe coefficient of thermal conductivity, no significant change wasobserved for Comparative Examples 8 to 10 and Examples 41 to 43.

[0211] As shown in Examples 41 to 43, it is preferable that thethickness of each of the second and third brazing members 30, 34 is notmore than 30 μm, and the thickness of each of the first and fourthbrazing members 26, 100 is not less than 30 μm.

[0212] Next, in the twelfth illustrative experiment, observation wasmade for the difference in peel strength, heat cycle, and coefficient ofthermal conductivity depending on the amount of the active element (Ti)in the first and fourth brazing members 26, 100 for Comparative Example11 and Examples 44 to 46. Experimental results are shown in FIG. 30.

[0213] As for the second and third brazing members 30, 34 used in theexperiment, the composition was Ag(59)-Cu(27.25)-In(12.5)-Ti(1.25), andthe thickness was 10 μm. As for the first and fourth brazing members 26,100, the composition was Ag(59)-Cu(27.25)-In(12.5)-Ti(1.25+a), for whicha Ti foil was used in combination with a foil of the brazing memberhaving the same composition as that of the second and third brazingmembers 30, 34. Therefore, the thickness was 10 μm (thickness of foil ofbrazing member)+thickness of Ti foil. The joining temperature was 780°C., and the joining time was 10 minutes.

[0214] In Comparative Example 11 and Examples 44 to 46, the ratiobetween the thickness of the brazing member foil and the thickness ofthe Ti foil was changed for the first and fourth brazing members 26,100. Specifically, as shown in FIG. 31, in Comparative Example 11, therewas given (thickness of brazing member foil): (thickness of Ti foil)=10(μm): 0 (μm). In Example 44, there was given (thickness of brazingmember foil): (thickness of Ti foil)=10 (μm): 0.7 (μm). In Example 45,there was given (thickness of brazing member foil): (thickness of Tifoil)=10 (μm): 1 (μm). In Example 46, there was given (thickness ofbrazing member foil): (thickness of Ti foil)=10 (μm): 2 (μm).

[0215] Experimental results are shown in FIG. 31. According to FIG. 31,when the Ti foil is not combined with the first and fourth brazingmembers 26, 100, i.e., when the amount of Ti is not increased, the peelstrength of the circuit board 10A is 32 kgf/cm² which is low. On theother hand, when the thickness of the Ti foil is increased to 0.7 μm, 1μm, and 2 μm as in Examples 44 to 46, the peel strength of the circuitboard 10A is also increased to 65 kgf/cm², 90 kgf/cm², and 115 kgf/cm².

[0216] No exfoliation was observed for any one of Comparative Example 11and Examples 44 to 46 in the heat cycle test of 500 cycles. As for thecoefficient of thermal conductivity, no significant change was observedfor Comparative Example 11 and Examples 44 to 46.

[0217] In the method for producing the circuit board 10A according tothe first embodiment, it is preferable that the sandblast treatment,which is performed in order to remove the conductive reactive layer 70exposed from the metal-removed portion of the circuit 36, is carried outby using the mask 82.

[0218] Accordingly, the surface of the circuit 36 (surface of thecircuit 36 or the plating layer 38) is not scraped by the grains for thesandblast treatment. Therefore, as for the specular reflection, it ispossible to maintain the specular reflection obtained at the stage atwhich the circuit 36 is formed or at the stage at which the platinglayer 38 is formed.

[0219] Therefore, for example, in the wire bonding treatment for thecircuit 36 (or the plating layer 38) to be performed thereafter, thereis no decrease in degree of tight contact of the bonding wire withrespect to the circuit 36 (or the plating layer 38).

[0220] Explanation will now be made for the thirteenth illustrativeexperiment to observe the relationship between the surface state(specular reflection) of the circuit 36 and the wire bonding performance(tight contact performance for bonding wire).

[0221] In the thirteenth illustrative experiment, observation was madefor the difference in tight contact performance for bonding wire withrespect to the circuit 36 (or the plating layer 38) depending on thespecular reflection of the circuit 36 (or the plating layer 38) forComparative Example 12 and Examples 47 to 53. Results are shown in FIG.32. In FIG. 32, the bonder output indicates the output obtained when thetight contact performance for bonding wire is satisfactory (acceptancerate: 100%).

[0222] In Comparative Example 12, when the conductive reactive layer 70exposed from the metal-removed portion of the circuit 36 was removed bymeans of the sandblast treatment, the sandblast treatment was directlyapplied to the surface of the circuit 36 (or the plating layer 38). InExamples 47 to 53, the sandblast treatment was performed using the mask82, in which the specular reflection of the surface was changedrespectively.

[0223] According to FIG. 32, the higher the specular reflection of thecircuit 36 (or the plating layer 38) is, the more satisfactory the tightcontact performance for bonding wire is, even when the bonder output islow. The lower the specular reflection of the circuit 36 (or the platinglayer 38) is, the more unsatisfactory the tight contact performance forbonding wire is. Practically, it is preferable that the surfaceroughness of the circuit 36 (or the plating layer 38) is not more thanRa=1.

[0224] As described above, in the circuit board 10A according to thefirst embodiment, the circuit 36 is manufactured by applying the etchingtreatment and the sandblast treatment to the metal plate 40 joined usingthe third brazing member 34 having the active element onto theinsulating substrate 32. Alternatively, the circuit-forming metal plate40, on which the circuit pattern 102 is previously formed, is joinedonto the insulating substrate 32 using the third brazing member 34, andthen the sandblast treatment is applied. Therefore, the etching residuesuch as the conductive reactive layer 70 or the like, which remains onthe insulating substrate 32, can be removed with ease. It is possible toobtain the circuit board 10A which is excellent in both of appearanceand characteristics.

[0225] Especially, the Ni plating layer 38 is allowed to remain on thecircuit 36. Therefore, the wettability of the solder layer 14 formed onthe circuit 36 is satisfactory. It is possible to reliably mount thesemiconductor device 16 on the circuit 36.

[0226] Next, explanation will be made with reference to FIG. 33 for acircuit board 10B according to a second embodiment and an electronicpart 12B using the circuit board 10B.

[0227] The electronic part 12B comprises a semiconductor device 16 whichis mounted on the circuit board 10B according to the second embodimentwith a solder layer 14 interposed therebetween, and a cooling fin 20which is fixed to the lower surface of the circuit board 10B with ametal layer 18 interposed therebetween, in the same manner as theelectronic part 10A described above.

[0228] The circuit board 10B according to the second embodiment isconstructed in approximately the same manner as the circuit board 10Aaccording to the first embodiment described above. However, the formeris different from the latter in that a metal plate 40, for which the Niplating layer 38 is not formed on the upper surface, is used.

[0229] Therefore, the method for producing the circuit board 10Baccording to the second embodiment is the same as the method forproducing the circuit board 10A according to the first embodimentdescribed above except that a resist 80 is formed on the metal plate 40,and then portions of the metal plate 40, which are exposed from windows80 a of the resist 80, are subjected to an etching treatment with anaqueous solution of ferric chloride or an aqueous solution of cupricchloride to form the circuit 36.

[0230] In this case, it is preferable that the sandblast treatment forthe circuit board 10B is performed under a condition in which thecircuit 36 remains on the insulating substrate 32 at a stage at whichthe conductive reactive layer 70 remaining at the metal-removed portion36 a of the circuit 36 is removed.

[0231] The embodiment described above is illustrative of the case inwhich the flat plate is used for the metal plate 18 joined to the lowersurface of the heat spreader member 22. Alternatively, as shown in FIG.34, a metal plate 18, which has a shape of fin, may be joined. Furtheralternatively, as shown in FIG. 35, the heat spreader member 22 itselfmay have a shape of fin.

[0232] The circuit board and the method for producing the same accordingto the present invention are not limited to the embodiments describedabove, which may be embodied in other various forms without deviatingfrom the gist or essential characteristics of the present invention.

[0233] As described above, according to the circuit board and the methodfor producing the same concerning the present invention, the etchingresidue including, for example, the conductive reactive layer, whichremains on the insulating substrate, can be removed with ease. It ispossible to obtain the circuit board which is excellent in both ofappearance and characteristics.

What is claimed is:
 1. A circuit board having a circuit on an insulatingsubstrate, wherein said circuit is formed by a sandblast treatment for ametal plate which is joined onto said insulating substrate and which hasa circuit pattern.
 2. The circuit board according to claim 1, whereinsaid circuit pattern is formed by etching said metal plate after saidmetal plate is joined onto said insulating substrate.
 3. The circuitboard according to claim 1, wherein said circuit pattern is formedbefore joining said metal plate onto said insulating substrate.
 4. Thecircuit board according to claim 1, wherein said circuit of said metalis joined onto said insulating substrate using a hard brazing membercontaining an active element.
 5. The circuit board according to claim 4,wherein said hard brazing member has a thickness of not more than 10 μmwhen said circuit of said metal is joined onto said insulatingsubstrate.
 6. The circuit board according to claim 4, wherein aconductive reactive layer is generated by a reaction between saidinsulating substrate and said active element in said hard brazingmember, and a part of said conductive reactive layer, which correspondsto a metal-removed portion of said circuit, is removed by said sandblasttreatment.
 7. The circuit board according to claim 4, wherein saidactive element is at lease one of elements belonging to any one of Group2A, Group 3A, Group 4A, Group 5A, and Group 4B in the periodic table. 8.The circuit board according to claims 1, wherein a plating layer isstacked on said circuit of said metal.
 9. The circuit board according toclaims 1, wherein a surface roughness of said circuit of said metal or asurface roughness of said plating layer on said circuit is not more thanRa=1.0 μm.
 10. The circuit board according to claim 1, wherein a heatspreader member or a heat sink member is joined to a lower portion ofsaid insulating substrate.
 11. The circuit board according to claim 10,wherein a buffer plate of metal is joined between said insulatingsubstrate and said heat spreader member or said heat sink member using ahard brazing member containing an active element; and a first joinedunit is provided, which comprises said circuit of said metal, saidinsulating substrate, said buffer plate, and said heat spreader memberor said heat sink member.
 12. The circuit board according to claim 11,wherein said heat sink member has a shape of fin.
 13. The circuit boardaccording to claim 11, wherein a metal plate is joined to a lowersurface of said heat spreader member or said heat sink member using ahard brazing member containing an active element; and a second joinedunit is provided, which comprises said circuit of said metal, saidinsulating substrate, said buffer plate, said heat spreader member orsaid heat sink member, and said metal plate.
 14. The circuit boardaccording to claim 13, wherein a ratio between a thickness of saidcircuit of said metal and a total thickness of said buffer plate andsaid heat spreader member or said heat sink member is 1:0.5 to 1:3 whena coefficient of thermal expansion of said insulating substrate in saidsecond joined unit is smaller than a coefficient of thermal expansion ofa material used for said heat spreader member or said heat sink member.15. The circuit board according to claim 13, wherein said second joinedunit is warped so that a lower surface of said metal plate isconvex-shaped toward the outside.
 16. The circuit board according toclaim 13, wherein said metal plate has a shape of fin.
 17. The circuitboard according to claim 11, wherein said buffer plate has a thicknessof 0.03 to 0.5 mm.
 18. The circuit board according to claim 11, whereinsaid insulating substrate has a thickness of not more than 0.6 mm whensaid insulating substrate is composed of Si₃N₄.
 19. The circuit boardaccording to claim 11, wherein said first or second joined unit has acoefficient of thermal conductivity of not less than 200 W/m.
 20. Thecircuit board according to claim 11, wherein said hard brazing memberhas a melting point of not more than 700° C.
 21. The circuit boardaccording to claim 11, wherein said hard brazing member is composed ofAg-Cu-In-Ti.
 22. The circuit board according to claim 11, wherein anamount of said active element in said hard brazing member for joining atleast said circuit of said metal and said insulating substrate is 0.05to 2%.
 23. The circuit board according to claim 11, wherein an amount ofsaid active element in said hard brazing member for joining said bufferplate and said heat spreader member or said heat sink member is 0.5 to10%.
 24. The circuit board according to claim 11, wherein a thickness ofsaid hard brazing member for joining said heat spreader member or saidheat sink member and said buffer plate, or a thickness of said hardbrazing member for joining said heat spreader member or said heat sinkmember and said metal plate is not more than 25% of a thickness of saidbuffer plate.
 25. The circuit board according to claim 11, wherein athickness of said hard brazing member for joining said insulatingsubstrate and said circuit of said metal, or a thickness of said hardbrazing member for joining said insulating substrate and said bufferplate is not more than 10% of a thickness of said buffer plate.
 26. Thecircuit board according to claim 25, wherein said thickness of said hardbrazing member is not more than 30 μm.
 27. The circuit board accordingto claim 10, wherein said heat spreader member or said heat sink membercontains at least one selected from the group consisting of SiC, AlN,Si₃N₄, BeO, Al₂O₃, Be₂C, C, Cu, Cu alloy, Al, Al alloy, Ag, Ag alloy,and Si.
 28. The circuit board according to claim 27, wherein said heatsink member is composed of a composite material in which an SiC basematerial is impregnated with Cu or Cu alloy.
 29. The circuit boardaccording to claim 28, wherein said heat sink member is composed of acomposite material in which a C base material is impregnated with Cu orCu alloy.
 30. The circuit board according to claim 1, wherein saidinsulating substrate is composed of AlN or Si₃N₄.
 31. A method forproducing a circuit board having a circuit of a metal plate on aninsulating substrate, said method comprising the steps of: joining saidmetal plate onto said insulating board using a hard brazing membercontaining an active element; and removing an electric conductive layeradjacent to a circuit pattern of said metal plate for partially exposingsaid insulating substrate to the outside.
 32. The method for producing acircuit board according to claim 31, said method comprising: a firststep of joining said metal plate onto said insulating substrate using ahard brazing member containing an active metal; a second step ofperforming an etching treatment for said metal plate to form saidcircuit pattern on said insulating substrate; and a third step ofexposing said insulating substrate by removing a conductive reactivelayer exposed from at least a metal-removed portion of said circuitpattern to obtain said circuit board having said circuit on saidinsulating substrate.
 33. The method for producing a circuit boardaccording to claim 31, said method comprising: a first step of formingsaid circuit pattern for said metal plate; a second step of joining saidmetal plate onto said insulating substrate using a hard brazing membercontaining an active metal; and a third step of exposing said insulatingsubstrate by removing a conductive reactive layer exposed from at leasta metal-removed portion of said circuit pattern to obtain said circuitboard having said circuit on said insulating substrate.
 34. The methodfor producing said circuit board according to claim 33, wherein saidfirst step includes a treatment for pressing said metal plate to formsaid circuit pattern of said metal plate.
 35. The method for producingsaid circuit board according to claim 34, wherein a bridge forconnecting parts of said circuit is also formed when said circuit isformed by pressing said metal plate; and said bridge is cut afterjoining said metal plate onto said insulating substrate.
 36. The methodfor producing said circuit board according to claim 35, wherein saidbridge is formed by half blanking said metal plate.
 37. The method forproducing said circuit board according to claim 35, wherein said bridgeis formed by etching for said metal plate.
 38. The method for producingsaid circuit board according to claim 31, wherein said metal plate isjoined onto said insulating plate at a temperature of not less than asolidus curve and not more than a liquidus curve of said hard brazingmember.
 39. The method for producing said circuit board according toclaim 31, wherein in said third step, said conductive reactive layer,which is exposed from said metal-removed portion of said circuitpattern, is removed by performing a sandblast treatment for an entiresurface including said circuit pattern to expose said insulatingsubstrate.
 40. The method for producing said circuit board according toclaim 31, wherein in said third step, said conductive reactive layer,which is exposed from said metal-removed portion of said circuitpattern, is removed by selectively performing a sandblast treatmentusing a mask to expose said insulating substrate.
 41. The method forproducing said circuit board according to claim 40, wherein in saidthird step, a plurality of masks are used, and a treatment, in whichsaid sandblast treatment is selectively performed using each of saidmasks, and then said conductive reactive layer exposed from windows ofsaid masks and exposed from said metal-removed portion of said circuitpattern is removed, is repeatedly performed with said plurality ofmasks.
 42. The method for producing said circuit board according toclaim 31, wherein said metal plate is composed of Cu, Cu alloy, Al, orAl alloy.
 43. The method for producing said circuit board according toclaim 31, wherein a metal plate, which has a plating layer formed on anupper surface, is used as said metal plate.
 44. The method for producingsaid circuit board according to claim 43, wherein said plating layer isan Ni plating layer.
 45. The method for producing said circuit boardaccording to claim 31, wherein said active element is at lease one ofelements belonging to any one of Group 2A, Group 3A, Group 4A, Group 5A,and Group 4B in the periodic table.
 46. The method for producing saidcircuit board according to claim 31, wherein said sandblast treatment insaid third step is performed under a condition in which at least saidcircuit pattern remains on said insulating substrate at a stage at whichsaid insulating substrate is exposed.
 47. The method for producing saidcircuit board according to claim 46, wherein a plating layer is formedon said circuit; and said sandblast treatment in said third step isperformed under a condition in which said plating layer remains on saidcircuit pattern at a stage at which said insulating substrate isexposed.
 48. The method for producing said circuit board according toclaim 31, wherein in said third step, said conductive reactive layer isgenerated by a reaction between said insulating substrate and saidactive element in said hard brazing member, and a part of saidconductive reactive layer, which corresponds to said metal-removedportion of said circuit pattern, is removed by said sandblast treatment.49. The method for producing said circuit board according to claim 48,wherein said sandblast treatment in said third step is performed under acondition in which at least said circuit remains on said insulatingsubstrate at a stage at which said part of said conductive reactivelayer corresponding to said metal-removed portion of said circuit isremoved.
 50. The method for producing said circuit board according toclaim 49, wherein a plating layer is formed on said circuit; and saidsandblast treatment in said third step is performed under a condition inwhich said plating layer remains on said circuit at a stage at whichsaid part of said conductive reactive layer corresponding to saidmetal-removed portion of said circuit is removed.
 51. The method forproducing said circuit board according to claim 31, wherein grains,which are finer than mesh #180, are used for said sandblast treatment.52. The method for producing said circuit board according to claim 51,wherein said grains are composed of Al₂O₃ or SiC.
 53. The method forproducing said circuit board according to claim 51, wherein an airpressure is 0.1 MPa to 0.25 MPa in said sandblast treatment.